version 6100 unsupported must be 7100 or later error!
As already answered (and promptly marked as off-topic) on StackOverflow...


  Try the Autodesk FBX Converter. Unfortunately it is no longer
  maintained, but it can converts many file formats, up to FBX 2013
  format.
  
  Alternatively, if you know a bit of C++ you can write a simple
  command-line file converter using the FBX SDK.


You can convert older formats to a compatible format using this utility and then import it into Blender.